1. Field of the Invention
The present invention relates to a semiconductor device, a display device, a liquid crystal display device, a driving method thereof, or a producing method thereof. Specifically, the present invention relates to a semiconductor device, a display device, or a liquid crystal display device including a driver circuit formed over the same substrate as a pixel portion, or a driving method of the devices. Alternatively, the present invention relates to an electronic device including the semiconductor device, the display device, or the liquid crystal display device.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have been actively developed. Specifically, a technique of forming a driver circuit such as a gate driver over the same substrate as a pixel portion by using a transistor formed using a non-single-crystal semiconductor has been actively developed because the technique makes a great contribution for reduction in cost and improvement in reliability.
Deterioration such as increase in threshold voltage or decrease in mobility is caused in the transistor formed using the non-single-crystal semiconductor in some cases. As the deterioration of the transistor advances, there is a problem in that the driver circuit becomes hard to operate and an image cannot be displayed. Accordingly, Patent Document 1 discloses a shift register which can suppress the deterioration of the transistor. Specifically, in FIG. 7 in Patent Document 1, two transistors are used for suppression of the deterioration in characteristics of the transistors. One transistor is connected between an output terminal of a flip-flop and a wiring to which VSS (hereinafter referred to as negative power supply) is supplied. The other transistor is connected between the output terminal of the flip-flop and a gate of a pull-up transistor. Then, during a period in which an output signal from the flip-flop is in an L level, the two transistors are alternately turned on. When the one transistor is turned on, VSS is supplied to the output terminal of the flip-flop through the one transistor. When the other transistor is turned on, VSS which is supplied to the gate of the pull-up transistor is supplied to the output terminal of the flip-flop through the other transistor. In this manner, the deterioration of the transistor can be suppressed. Further, since VSS is always supplied to the output terminal of the flip-flop, the output signal from the flip-flop is easily maintained as the L level.